CHECK-RAS Exercise

FW EM 02: Encroachment Method ___is used.

CHECK-RAS will print an FW EM 02 message if Encroachment Method 2 or 3 is used at a cross section when running the Floodway Check option in CHECK-RAS.

An example of an FW EM 02 message is provided in this month's bulletin when Encroachment Method 3 is specified at a cross section. Encroachment Method 3 calculates encroachment stations for a specified percent reduction in the conveyance of the natural profile for each cross section. The natural profile in the flood insurance study represents the 1-percent annual chance flood profile. Encroachment Method 3 has two options to reduce the conveyance; equal conveyance reduction option and proportionate conveyance reduction option.

Encroachment Method 3 requires that the first profile must be a natural (un-encroached) profile. If equal conveyance reduction option is selected it will reduce the conveyance from each overbank of the first profile half of the specified percent reduction. For example, a value of 10 percent would indicate that 10 percent of the conveyance based on the natural profile (first profile) will be eliminated by removing 5 percent from each overbank.

If equal conveyance reduction option is not selected it will reduce the conveyance proportionate to the overbanks conveyances of the natural profile. For instance, under natural profile, if the left overbank has twice as much conveyance as in the right overbank, a 10 percent conveyance reduction value would reduce 6.7 percent from the left overbank and 3.3 percent from the right overbank./

Encroachment Method 4 is based on the concept that the conveyance for the encroached cross section (at some higher elevation) is the same as the conveyance of the natural cross section at the natural water level. First, a target surcharge value is added to the natural water level to find the conveyance of the entire cross section. The additional conveyance above the natural water level is reduced equally from each overbank so that the conveyance between the encroachment stations will be the same as the conveyance under the natural water level.

Since Encroachment Method 3 does not compute the encroachment stations the same as Encroachment Method 4 and in some instance it will compute the encroachment stations based on the proportionate reduction in conveyance, this method should not be used in flood insurance studies. This bulletin compares the results between Encroachment Method 3 and Encroachment Method 4. The proportionate conveyance reduction method will be used for Encroachment Method 3.

Please download the project file, plan files, geometric file, and flow file, which are available in the WinZip archive referenced here, and unzip the files to one directory on your local computer.

Problem

Open the project file (2FWEM02.prj) with the HEC-RAS program and select Plan 01 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Click on the Option menu from the 'Steady Flow Analysis' window and select Encroachments. The 'Encroachments' window will appear as shown in the figure below. The figure below shows that Encroachment Method 3 was specified with 16 under the Value 1 column at all cross sections. The number 16 is the percentage to reduce the conveyance from the natural profile. Please note that 'Equal Conveyance Reduction' box at the top of the window is not checked. That means the proportionate conveyance reduction method will be used by HEC-RAS to compute the encroachment stations. Close the Encroachments window and run HEC-RAS by clicking on the 'Compute' button. After the computation is done, close the 'Steady Flow Analysis' window.

Click the 'Cross Sections' button from the HEC-RAS main window. The 'Cross Section' window will appear as shown below. As an example, select RS 56.97. Since the proportionate conveyance reduction method is used, only some portion of the left overbank area below the natural water-surface elevation is being encroached. Therefore, some portions of the left overbank area will be in the floodway.

When CHECK-RAS is run for this plan, it will provide a Summary Table and a message regarding the use of Encroachment Method 3. Close all windows except the main HEC-RAS window. Generate a report file from the 'File' menu. After generating the report file for use with the CHECK-RAS program, please save the project.

Open the saved project (2FWEM02.prj) in the CHECK-RAS program and extract the data. The 'Select Profile' window will be displayed. elect the 100-year Profile and the Floodway Profile and click on the OK button. Then click on the Floodway Check button on the program menu bar. When prompted with the message: "You need to specify allowable surcharge..." click the 'Yes' button. Click on the Report button on the program menu bar and scroll down to the ENCROACHMENT METHOD CHECK location in the displayed window. The following figure shows the FW EM 02 messages at the ENCROACHMENT METHOD CHECK location.

If you highlight the message number FW EM 02, the following message box will appear.

The Help message provides suggestions to resolve the problem. The Help message suggested using the equal conveyance reduction method, which is Encroachment Method 4 in HEC-RAS.

Solution

Close the CHECK-RAS window and open the project file (2FWEM02.prj) with the HEC-RAS program. Select Plan 02 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Click on the Option menu from the 'Steady Flow Analysis' window and select 'Encroachments'. The 'Encroachments' window will appear as shown in the figure below. The following figure shows that Encroachment Method 4 was specified at all cross sections.

Close the 'Encroachments' window and run HEC-RAS by clicking on the 'Compute' button. After the computation is done, close the 'Steady Flow Analysis' window. Click the 'Cross Sections' button from the HEC-RAS main window. The 'Cross Section' window will appear as shown below. As an example, select RS 56.97.

The figure above shows that by using the equal conveyance reduction method the left encroachment station is set at the left channel bank station and the left overbank area is completely outside of the floodway.

The following steps are followed by HEC-RAS when the equal conveyance reduction method is used:

  1. The target surcharge values specified for the floodway profile in the 'Encroachments' window at each cross section are added to the water-surface elevation of the same cross section from the first profile. Then the conveyance is computed at each cross section for the increased water-surface elevation of the floodway profile.
  2. Half of the difference in conveyance between the first profile and the floodway profile at a cross section is subtracted from the left overbank area of the cross section of the floodway profile to determine the left encroachment station. The other half is subtracted from the right overbank area of the cross section of the floodway profile to determine the right encroachment station.
  3. If half of the difference in conveyance cannot be obtained from one overbank, the difference will be made up, if possible, from the other overbank, except that the encroachments will not be allowed to fall within the channel portion.
  4. After the encroachment stations are determined from the above procedure, HEC-RAS will compute the true floodway water-surface elevations of the floodway profile for the encroached cross sections based on the energy equation. The difference in water-surface elevations between the first profile and the floodway profile is the computed surcharge value. The computed surcharge value may or may not be the same as the target surcharge value specified in the 'Encroachments' window.

The results between Encroachment Method 3 and Encroachment Method 4 can be compared from the Profile Output Table window. From the main HEC-RAS window, click on the 'Profile Table' button. The 'Profile Output Table' window will be displayed. Create a user-defined output table including the variables, Q Total, WS. Elev, Prof Delta WS, Conv. Left, Conv. Chnl, and Conv. Right, as shown below. To select different plans, from the Profile Output Table window, click on the Options menu and select Plans. The 'Select Plan' window will be displayed. Select Encroachment Method 3 (Plan 01) and Encroachment Method 4 (Plan 02) and click on the OK button. The information about the two plans will be shown in the user-defined output table.

The first two lines are the information from the natural profile for each encroachment method and the second two lines are from the floodway profile. Since Encroachment Method 3 uses the proportionate conveyance reduction method, not all of the conveyance is reduced from the left overbank. However, Encroachment Method 4 reduces all of the conveyance from the left overbank because half of the conveyance amount to be reduced is equal to or larger than the conveyance value at the left overbank.

View the archive page for all Flood Hazard Mapping listservs.

Last Modified: Friday, 15-Jun-2007 11:57:20 EDT